Thermochemical Routes for the Production of Advanced Biofuels 

Thermochemical processes are characterized by the conversion of one molecule to another, through the action of pressure, temperature and chemical agents, providing breaking and formation of chemical bonds. There are several thermochemical processes, among them pyrolysis, gasification, and hydrothermal liquefaction.

Gasification is a process for converting solid and/or liquid carbonaceous materials, through partial oxidation at high temperatures (700-1000 ^oC), into a flammable gaseous mixture containing carbon monoxide and hydrogen, as well as methane, carbon dioxide, nitrogen, and light hydrocarbons. The exact composition of the final gaseous mixture, called synthesis gas, will depend on the final product to be obtained through, for example, the Fischer-Tropsch synthesis and, consequently, on the Cleaning and Conditioning System used for the raw gas mixture from gasification.

The Gasification Gas Cleaning System may or may not contain the Catalytic Tar Reforming and Water-Gas-Shift Reaction (WGS) Process(es). The Gas Cleaning System is composed of the following sequence of steps: (i) coarse particulate removal (sand filter); (ii) tar cracking (fixed bed reactor with dolomite); (iii) removal of tars, particulates and alkali metals (Venturi scrubber); (iv) HCl and NH₃ removal (mist scrubber); (v) H₂S and HCl removal (activated carbon reactor) and (vi) fine particulate removal (ceramic filter).

Catalytic Tar Reform is a catalytic process at high temperatures (500-1000^oC) in the presence of steam and O₂ (autothermic reforming) to remove the tar – a condensable organic fraction of the gasification product consisting mainly of aromatic hydrocarbons larger than benzene. It is necessary to avoid fouling and unwanted reactions in downstream processes and to increase the formation of synthesis gas (CO + H₂).

The Water-Gas-Shift Reaction (WGS) is a catalytic process at moderate temperatures (200-400^oC) in the presence of water vapor for the conversion of carbon monoxide to hydrogen and carbon dioxide (CO + H₂O ⇔ H₂ + CO₂). This operation is necessary to increase the amount of H₂ in order to adjust the composition of the synthesis gas appropriate for Fischer-Tropsch Synthesis (H₂/CO = 2) and to proceed with the hydroprocessing reactions.

Fischer-Tropsch Synthesis (SFT) is the reaction for producing long-chain hydrocarbons from CO and H₂ (synthesis gas) at low temperatures 200 to 300^oC and moderate pressures 10 to 50 bar. It allows the formation of liquid hydrocarbons with high added value, such as olefins, gasoline, kerosene, and diesel, directly or coupled to catalytic hydroprocessing.

Fast Pyrolysis is an endothermic process of conversion of solid carbonaceous materials, in the absence of oxygen, at moderate temperatures (400-600^oC), at a residence time of around 2 seconds. Bonds in the carbonaceous structure of the biomass are destroyed by heat and recombined rapidly, resulting, after rapid condensation, in the formation of a predominantly liquid mixture, bio-oil, containing various oxygenated compounds.

Autothermic or Fast Oxidative Pyrolysis occurs in the presence of a small amount of oxygen to provide the heat required for Pyrolysis. The bio-oil formed in pyrolysis can be submitted to Gasification (pure or blended with pyrolysis biochar), to Hydrodesoxygenation (HDO) or to Co-processing in FCC Units for the production of advanced biofuels.

Hydrothermal Liquefaction (HTL) is a process for converting solid carbonaceous materials in the absence of oxygen at moderate temperatures (105-400^oC) and pressures (2 to 20 MPa), usually in a batch reactor. Bonds in the carbonaceous structure of the biomass are destroyed and recombined resulting in the formation of a predominantly two-phase liquid mixture, aqueous and organic (bio-oil) containing various oxygenated compounds. The bio-oil formed can be further gasified, coprocessed, or refined (HDO) to produce advanced biofuels.

Hydrodeoxygenation (HDO) is one of the refining processes for pyrolysis and HTL bio-oils that provides the removal of oxygen from the organic molecules, with the formation of H2O. This process is used to stabilize the physicochemical properties of the bio-oil and/or to obtain molecules with higher calorific value and/or “drop-in” fuels. It is a catalytic process carried out at moderate temperatures (200 to 400^oC) and pressures that can range from atmospheric to high (around 200 bar). The catalysts used are based on the traditional ones employed in the HDT processes (hydrotreatment), that is, NiMoS and NiWS, or those based on Ni, Fe or noble metals such as Pt.

Coprocessing is the oil refining process, where bio-oil or other renewable feedstocks, after being added (usually small amounts) to vacuum residue and other oil processing streams are converted (catalytic cracking) to “drop-in” fuels such as gasoline, kerosene, and diesel.

Fundamental Bibliographical References

Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power, Second Edition

Biomass Gasification and Pyrolysis: Practical Design and Theory 2nd Edition (2013)

Review of fast pyrolysis of biomass and product upgrading

A comparative techno‐economic assessment of three bio‐oil upgrading routes for aviation biofuel production